I think the answer is "tradition", like you said. It's a little creepy to me, but some of these "remains" were not complete - meaning not all the body parts could be accounted for. Some bones may be where they are said to be - though after all this time, there will be nothing but ashes left - but there was a lucrative market for relics and the more prestigious and wealthy a church was, the better chance they got to get "dibs" on the important relics. Some small insignificant church may only have been able to get Saint So-and-so's baby toe. It's no surprise that Rome laid claim to most of the "biggies". It borders on the macabre, to me, because people come to venerate these remains and relics as if they had supernatural powers. At the resurrection, it's a good thing God knows who is where, that's all I can say. ;o)
I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held (Apocalypse/Revelation 6:9)
Several years ago I venerated the relics of Virgin Mary's parents, Joachim and Anna, in Rocklin, CA. They were fragments of bones. Provenance of some relics cannot be ascertained; so there is an index finger of St. John the Baptist in Nelson-Atkins museum in Kansas City. No one knows if it ended up in a museum rather than in a church because some Protestant vandals stole it and destroyed the records, or the Muslim vandals did the same, or it is not genuine. The same can be said of the head of St. John the Baptist in a mosque in Damascus, and his right hand venerated in Russia. When a Protestant sneers at the Holy Relics, there is a special revulsion in the Christian soul as often it is the very horror of the Reformation that destroyed the proofs of authenticity. It will be a glorious day when the demons of both the Islam and the Reformation run off and we forget of the evils they did to the Christendom.
Yes, I thought there were a lot of bits and pieces here and there. It puts in mind a comment by Mark Twain after a trip through the holy land...something to the effect that they had seen enough fragments of the “true cross” to build a cathedral. It seems to me these carcasses (or fragments thereof) are of little value other than as props for an organization or individual church to use in laying claim to the faith and any inherent authority resulting from that claim. That’s why I’m suspicious. It’s very convenient, and no doubt very powerful in an age of of pervasive illiteracy and superstition.